Obesity is a highly prevalent disease that causes significant morbidity and mortality in the population of the United States. Nonetheless, there exists a limited understanding of the contributions of various aspects of energy metabolism to the pathogenesis of the disease. We propose to examine resting metabolic rate, daily energy expenditure, and the thermogenic and hormonal responses to overfeeding in two separate studies. In the first study, daily energy expenditure in 30 obese adolescents will be compared to 30 non-obese adolescents matched for age and sex. Mean daily C02 production will be measured by the newly developed doubly labeled water (2H2180) method. Oxygen consumption will be calculated from RQ estimated from records of dietary intake. Comparisons of the two groups will permit a more definitive assessment of the role of reduced energy expenditure as an etiologic factor in the onset or persistence of the disease. Stepwise linear regression of anthropometric measurements with simultaneous measurements of body composition will be used to determine the best anthropometric index to determine body fat and lean body mass (LBM). In the second study, 10 obese and non-obese adolescents will be overfed 1500/kcal/day above their usual caloric intake for 14 days. The surfeit colories will be predominantly carbohydrate. Mean daily energy expenditure will be measured by doubly labeled water during the period of surfeit feeding. Mean RQ will be estimated from frequent measurements of RQ by indirect calorimetry throughout the study. Changes in resting metabolic rate or the thermogenic response to a meal containing surfeit calories will be measured at four different intervals by indirect calorimetry. Prior to the study and 14 days after surfeit feeding, the hormonal response to a meal will be measured and compared in the same individual and between groups of obese and non-obese adolescents. Changes in body composition, and comparisons of these aspects of energy expenditure will help to clarify whether obese adolescents are capable of dissipating excess carbohydrate calories in a fashion comparable to the non-obese.